Find first set
family of related bitwise operations on machine words
In computer software and hardware, find first set (ffs) or find first one is a bit operation that returns the index or position of the least significant 'one' bit in its argument, counting from the least significant bit. A related operation is count trailing zeros (ctz) or number of trailing zeros (ntz), which counts the number of zero bits following the least significant one bit. The operation that finds the index of the most significant set bit is log base 2, so called because it computes the binary logarithm ⌊log2(x)⌋. This is closely related to count leading zeros (clz) or number of leading zeros (nlz), which counts the number of zero bits preceding the most significant one bit.
There are two common variants of find first set, the POSIX definition which starts indexing of bits at 1, herein labelled ffs, and the variant which starts indexing of bits at zero, which is equivalent to ctz.
Most modern CPU instruction set architectures provide one or more of these as hardware operators; software emulation is usually provided for the others, either as compiler intrinsics or in system libraries.
Examples
Given the following 32-bit word:
0000 0000 0000 0000 1000 0000 0000 1000
The count trailing zeros operation would return 3, while the count leading zeros operation returns 16. The count leading zeros operation depends on the word size: if this 32-bit word were truncated to a 16-bit word, count leading zeros would return zero. The find first set operation would return 4, indicating the 4th position from the right. The truncated log base 2 is 15.
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